cubicweb #2265710 [devtools] xvfb-run is too slow [validation pending]

it waits 3s before starting the command each time

priorityminor
typebug
done in3.14.7
load0.100
load left0.000
closed by#4d76492508f3 [devtools] make xvfb-run not wait 3s each time (closes #2265710)